The present invention relates to an apparatus for scanning optical recording media, having a detection unit, an evaluation unit and a connection between these units.
In apparatuses of this type, voltage coupling is usually provided between detection unit and evaluation unit. This has the disadvantage that the two units have to operate approximately in the same voltage range.
An object of the present invention is to propose an apparatus of the generic type in which it is not necessary for detection unit and evaluation unit to operate in the same voltage range.
This object is achieved according to the invention by virtue of the fact that the evaluation unit has a controller, which holds the input of the connection at the value of a reference voltage of the detection unit or a value which is proportional thereto. This has the advantage that current coupling is made possible, even though the detection unit has a voltage output, and an accurate transmission of the signal is nevertheless made possible, since both the detection unit and the evaluation unit operate with the same voltage reference with a voltage reference derived from a single reference voltage. According to the invention, the voltage references utilized by detection unit and evaluation unit can be chosen either such that they are identical or alternatively such that they have different magnitudes.
According to the invention, the connection has a defined resistor and the control loop of the evaluation unit uses a reference voltage of the detection unit as desired value and the signal present on the connection as actual value. This has the advantage that the control loop regulates the voltage present on the connection to a constant value, with the result that the input current can be evaluated by the evaluation unit. In this case, the defined resistor is chosen in such a way as to obtain optimum matching between detection unit and evaluation unit. It is to be noted that the desired value is sometimes also referred to as the set point of a servo control loop.
In accordance with an advantageous refinement, the evaluation unit has a differential input. This reduces the influence of interference on the supply voltage or other equivalent or common-mode interference, since, in this case, the reference value is also present as a current and, consequently, two current values are compared with one another. Instances of common-mode interference that possibly occur affect both currents and therefore cancel one another out to the greatest possible extent during evaluation. The accuracy of the signal to be evaluated is thus increased.
A particularly simple realisation of a differential input consists in the evaluation unit having a further control loop, whose desired value corresponds to the reference voltage and whose actual value is connected to the reference voltage via a defined resistor corresponding to that of the connection.
The evaluation unit advantageously has an adjustable current amplifier. Depending on the type of further processing, the corresponding output current of the current amplifier is then fed to an analogue current or current-voltage conversion output stage and/or to an analogue-digital converter. Consequently, in an advantageous manner, even if the input current is small, a large output signal is nevertheless available.
The gain factor of the current amplifier is variable according to the invention. This has the advantage that it can be matched to the respective conditions of the detector unit, of the optical arrangement, of the connection and, if appropriate, of other parts of the apparatus. In an advantageous manner, the gain factor is also variable during operation, in order to be matched for example to changes which are due to ageing, dictated by temperature, dependent on properties of the recording medium or else caused in some other way.
According to the invention, the defined resistor can also be chosen within wide limits. A configuration of the defined resistor as a variable resistor also lies within the scope of the invention. This has the advantage that, in combination with the variable current gain of the evaluation unit, it is possible to use a broad spectrum of different detector units with different sensitivities and recording media having different reflectivity, without adaptations to the hardware being necessary. A further advantage of this configuration is that addition of different signals from the detection unit can be obtained simply by connecting a single input of the evaluation unit to a plurality of outputs of the detection unit via a respective defined resistor.
One variant of the invention provides for the evaluation unit to be connected to a reference voltage source via a voltage divider. This has the advantage that the reference voltage present at the evaluation unit can be reduced even more extensively, without requiring changes, in particular a reduction in the reference voltage of the detection unit. Consequently, the evaluation unit can be operated with an even lower supply voltage and hence more economically.
A further variant of the invention provides for the evaluation unit itself to have a reference voltage source. This has the advantage that the reference voltage source can be implemented in a simple and cost-effective manner in the integrated circuit of the evaluation unit. The reference voltage generated in this case is not only utilised internally but also, advantageously having been boosted by means of an external voltage boosting circuit, is used as a reference voltage for the detection unit.